The present invention relates to an ion source, which can be used, for example, in the analysis of gases of mass spectrometry.
An ion source of a first type is known, which comprises an ionization chamber, an electron source constituted by a heating filament (cathode) and a trap (anode) facing it.
The electrons emitted are accelerated between the filament and the ionization chamber and ionize the molecules of the gas contained in the chamber. A servo system may optionally permit, as a result of the electron current collected on the anode, the regulation of the current circulating in the filament, so that it can stabilize the electron flux emitted towards the ionization zone.
A magnetic field in the direction of the electron beam channels the electrons and permits a better extraction of the ions produced towards an analyzer, such as e.g. a mass spectrometer.
In this type of source, each electron emitted only traverses the ionization chamber once when it does not ionize a molecule. This leads to a low ionization efficiency between 10.sup.-4 and 10.sup.-6. This efficiency is defined by the ratio of the number of ions formed to the number of electrons emitted.
Another coefficient characterizing the performances of an ion source is defined by the ratio of the number of ions formed to the number of molecules introduced, said coefficient being called "luminosity". The luminosity of the aforementioned sources is very low.(-10.sup.-5).
Ion sources of a second type are known, whose ionization efficiency and luminosity are higher than those of the aforementioned sources. These sources comprise a filament, which produces electrons, an accelerating cathode and an anode collecting the electron current. An intermediate electrode is positioned between the cathode and anode and an anticathode is positioned behind the anode. Voltage pulses are applied to the cathode, so as to bring about a discharge between the cathode and the intermediate electrode and this discharge ionizes the gas. The electrons produced then oscillate in the zone between the intermediate electrode and the anticathode, in which a potential trough is produced. The electrons produce an ionization of the gas in said zone. This source has a better efficiency of luminosity than the aforementioned source, but its construction is complicated and it is very difficult to realize.